An optical comb source is a coherent multi-carrier signal with equal spacings between adjacent carriers. Optical frequency comb sources have recently stirred a lot of interest due to wide ranging applications such as optical arbitrary waveform generation, photonic microwave signal generation, optical signal processing and multicarrier spectrally efficient transmission techniques with the sub-channel spacing equal to the symbol rate of each sub-channel. Optical frequency comb sources with good spectral flatness, stability, low linewidth and wavelength flexibility are highly desirable for such applications.
One of the conventional approaches used in realizing a comb source is based on mode-locked semiconductor lasers (MLL), for which an example is A. Akrout, A. Shen, R. Brenot, F. Van Dijk, O. Legouezigou, F. Pommereau, F. Lelarge, A. Ramdane and G-H. Duan, “Error-free transmission of 8 WDM channels at 10 Gbit/s using comb generation in a quantum dash based mode-locked laser,” ECOC, Th3.D.4, September 2008. Although this technique can generate multi-carrier signals spanning over a wide bandwidth, it inherently suffers from cavity complexity and does not offer the free spectral range (FSR) tunability since the comb line spacing is fixed by the cavity length of the laser. Moreover, the optical linewidth of the individual comb lines can be relatively large (several MHz) preventing higher order (or low baud rate) advanced modulation formats to be imposed. More recently wavelength tunable comb generation by use of external optical modulators has been reported where the line spacing and the central wavelength of the comb can both be varied [T. Sakamoto, T. Kawanishi and M. Izutsu, “Widely wavelength-tunable ultra-flat frequency comb generation using conventional dual-drive Mach-Zehnder modulator,” Electron. Lett. 43(19), 1039-1040 (2007). However, the large insertion loss of the modulator (especially when cascaded) coupled with the modulation efficiency can prove prohibitive. Another technique employed, as described in S. Bennett B. Cai, E. Burr, O. Gough, and A. J. Seeds, “1.8-THz Bandwidth, Zero-Frequency Error, Tunable Optical Comb Generator for DWDM Applications,” IEEE Photon. Technol. Lett., 11(5), 551-553 (1999) entails the use of phase modulation in an amplified fiber loop with the main disadvantages being poor spectral flatness and the limited optical linewidth of the laser source in order to suppress Brillouin induced instabilities in the comb spectra.
The present application is directed at providing an alternative to these techniques.